Electroluminescent devices for use in electroluminescent displays, such as flat panel displays, are well known in the industry and are set forth in U.S. Pat. No. 5,247,190 to Friend et al.; U.S. Pat. No. 5,682,043 to Pei et al.; U.S. Pat. No. 5,723,873 to Yang; as well as Baigent et al. (1994), “Conjugated Polymer Light-Emitting Diodes on Silicon Substrates,” Appl. Phys. Letter, 65(21):2636-38. Electroluminescent devices, therefore, may be fabricated upon a suitable substrate in a multilayer thin-film configuration, such as, for example, wherein a layer of electroluminescent material is positioned between an electron-injection and a hole-injection electrode layer in a “sandwich” configuration. When a voltage gradient is applied across the electrode layers, holes and electrons are injected into the electroluminescent material from the hole-injection and electron-injection electrode layers, respectively, which results in the emission of light through one or more of the electrode layers when the holes and electrons are combined in the electroluminescent material.
Commonly, electroluminescent devices for use in electroluminescent displays, such as those disclosed above, include a cathode and/or electron-injection layer that has been fabricated from a low work function metal, such as a calcium or barium metal. Low work function metals are typically used in the production of electroluminescent displays because they facilitate the injection of electrons into the electroluminescent material.
Using low work function metals, such as barium or calcium, as a cathode and/or electron-injection layer in electroluminescent devices, however, suffers from several drawbacks. For instance, one drawback is that low work function metals are typically very chemically reactive, unstable in air, and rapidly absorb moisture from the ambient environment thereby producing hydrogen gas and the corresponding metal hydroxide. This reaction results in a loss of the ability of the metal cathode and/or electron-injection layer to inject electrons into the electroluminescent material as the metal is oxidized, and consequently results in a loss of function of the electroluminescent device in which the low work function metal cathode/electron-injection layer is employed. Accordingly, in order for the device to have a reasonably long lifetime, these devices are traditionally fabricated and encapsulated in an inert, gas environment, which increases the complexity and expense of fabrication.
An additional drawback is that the deposition process by which the metal is deposited on a suitable substrate, so as to produce a low work function cathode layer and/or electron-injection layer, typically involves a high vacuum evaporation process. A high vacuum evaporation process is disadvantageous because the metal to be deposited must first be evaporated and then deposited in a vacuum, which process is complex, time consuming, and expensive.